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Some animals are widely known for resorting to a different way of obtaining food: theft. Species that steal for eating are known as kleptoparasites. Usually they choose other species as victims, but that's not always true. Some species, like the kelp gull (Larus dominicanus), perform intraspecific kleptoparasitism. Each time a gull goes foraging it may choose between being a fisher, and get it's own food, or being a pirate, and steal another gull's food. The main reason that may take a gull into being a pirate is that, in some occasions, it may be easier to steal. But there are many factors that affect the success of an stealing attempt, and sometimes being a pirate can be costlier than being a fisher.

One factor that affects the success of pirates is how many of them exist in the population. If there are few pirates and many fishers, there might be many gulls to steal from, making the theft easier. Otherwise, if there are many pirates and few fishers, there may be competition between pirates, and finding a gull to steal from may be hard.

Several other factors influence the choice of a gull. Food availability may also be a factor. If there is plenty of fish, then fishing may be very easy. Also, the age of a gull may play an important role. Juvenile gulls tend to have less experience on fishing and will be a pirate more frequently. The handling time of the fish may also be important. If it takes too long to handle, the fishers may be more susceptible to a pirate.

Build a mathematical model that describes the number (or proportion) of fishers and pirates on a population of kelp gulls taking into account the factors that affect the choice of the gulls.

• Is there an optimal proportion of pirates?
• Are there conditions in which no gull wants to be a pirate?

Steele, W. K., & Hockey, P. A. (1995). Factors influencing rate and success of intraspecific kleptoparasitism among kelp gulls (Larus dominicanus). The Auk, 847-859.